In recent years, characteristics of organic electronic materials have been remarkably improved. In particular, the so-called organic bistable materials where, if a voltage is applied to the material, an electric current in a circuit rapidly increases at a voltage above a certain value and a switching phenomenon is observed have found application for switching elements for driving the organic EL display panels or for high-density memories.
FIG. 8 shows an example of a voltage-current characteristic of an organic bistable material demonstrating the above-described switching behavior.
As shown in FIG. 8, the organic bistable material has two current-voltage characteristics: a high-resistance characteristic 51 (OFF state) and a low-resistance characteristic 52 (ON state), and a nonlinear response characteristic such that if a bias Vb is applied in advance and then the voltage is raised to a threshold voltage (Vth2) or higher, a transition is made from the OFF state to the ON state, and if the voltage becomes equal to or less than another threshold voltage (Vth1), then a transition is made from the ON state to the OFF state and the resistance value changes.
In other words, the so-called switching operation can be performed by applying a voltage of Vth2 or higher or Vth1 or lower to the organic bistable material. Here, Vth1, Vth2 can be also applied as pulse voltages.
A variety of organic complexes are known as the organic bistable materials demonstrating such nonlinear response. For example, R. S. Potember et al. fabricated a switching element having two stable resistance values with respect to a voltage by using a Cu-TCNQ (copper-tetracyanoquinodimethane) complex (R. S. Potember et al. Appl. Phys. Lett. 34, (1979) 405).
Further, Kumai et al. used a single crystal of a K-TCNQ (potassium-tetracyanoquinodimethane) complex and observed the switching behavior caused by the nonlinear response (Kumai et al. Kotai Butsuri, 35 (2000) 35).
Furthermore, Ando et al. formed a thin film of a Cu-TCNQ complex by using a vacuum vapor deposition method, clarified switching characteristics thereof and investigated the possibility of application to an organic EL matrix (Ando et al. Preprints of Applied Physics Association Conference, Spring 2000, Vol. 3, 1236).
Further, with respect to the charge-transfer complexes composed of the above-described two components, Yang et al. obtained with an organic film composed of a single component a switching characteristic similar to that of the charge transfer complexes by sandwiching a very thin Al film with 2-amino-4,5-imidazole dicarbonitrile thin films (Yang Yang et al. Appl. Phys. Lett. 80, (2003) 362). With such configuration, because each complex is a thin film composed of a single component, the composition controllability is significantly improved over that of the conventional charge transfer complexes of two-component systems.
However, the above-described switching element using an organic material composed of a single molecule has the following problems.
Thus, the voltage Vth2 value that causes the transition from the OFF state to the ON state shown in FIG. 8 has a large spread and the characteristic is not stable. The cause of this spread has not yet been clarified. Apparently, very small peaks and valleys are present on the interface of the organic material layer and metal electrodes, and when a transition from the OFF state to the ON state occurs, an electric charge is injected into the organic material film due to electric field concentration on those peaks and valleys. At this time, the peaks and valleys on the interface are determined by the flatness of the organic material film, but because those very small peaks and valleys are difficult to control, the spread of Vth2 values occurs inevitably.
Further, when this switching characteristic is applied to driving other elements, a Vth2 value exceeding the operation voltage of the element to be driven is required for the organic bistable element. The problem is that, for example, in organic EL display panels and the like, this value has to be 10 V or higher, but in the above-described conventional switching elements the Vth2 value is as low as 3–5 V.
Moreover, in applications for organic EL display panels, memories, and the like, the above-described bistable material is usually used in a simple matrix configuration. In such simple matrix configuration, electrodes are locally formed as a pattern on an insulating substrate (usually, a glass substrate is used), the insulating substrate and electrodes appear alternately as a prime layer, and the above-described organic bistable material is formed, for example, by vapor deposition on the substances of two different types.
Here, the above-described organic bistable material is in the form of molecules with a comparatively low molecular weight that have electron-donating groups and electron-accepting groups, and a specific feature thereof is that they easily grow as grains because polarization is high. As a result, though a dense film is formed as small grains on the metal electrodes, larger grains are easily formed on the flat glass substrate having an amorphous structure.
For this reason, the problem associated with the conventional simple matrix configuration is that because the organic bistable film present on the glass substrate forms large grains, a large leakage current is generated on grain boundaries of the organic bistable film present on the glass substrate adjacent to the electrodes and this current increases the OFF current, thereby greatly reducing the ratio of the ON current of switching to the OFF current.
The present invention was created to resolve the above-described problems of the conventional technology and it is an object thereof to obtain an organic bistable element With a large Vth2 value, a small spread, and a large ratio of the ON current to the OFF current in a switching element in which an organic bistable material is disposed between two electrodes.